This invention relates generally to electronic components and more particularly concerns low profile surface mountable electronic components having an improved structure for increasing the performance of the component.
Over the last decade the electronics industry has made many advances with respect to electronic components. One of the more significant advances was the introduction of the Surface-Mount Device (SMD) or surface mount technology. SMDs allow electrical components to be mounted on one side of a PCB, without requiring the leads of the components to be inserted through the printed circuit board (PCB) and soldered to the reverse side of the PCB, (i.e., an older method of mounting components to PCBs referred to as through-hole technology). An SMD component has small metalized pads (terminals or leads) connected to its body, which correspond to solder pads (or lands) located on the surface of the PCB. Typically the PCB is run through a solder-paste machine (or screen printer), which puts a small amount of solder on the solder pads of the PCB. Then, the component is placed on the PCB, and the PCB is sent through a re-flow oven to heat the solder paste and solder the component leads to the PCB solder pads. The primary advantage to this technique is that both sides of the PCB can now be populated by electronic components. Meaning one PCB today can hold an amount of electrical components equal to two PCBs in the past.
As a result of this advancement in technology, the size of electronic circuits has decreased, thereby enabling smaller electronic devices to be manufactured. Current electronic circuits are mainly limited by the size of components used on the PCB. Meaning, if the electronic components can be made smaller, the circuits themselves can be made smaller as well. Unfortunately, there are some electronic components that have been more difficult to configure for SMD technology. For example, over the years many advances have been made in creating surface mount single winding components such as inductors. To date, however, there have only been minimal advances with respect to multi-winding components such as transformers. This is, at least in part, due to the difficulty in obtaining high quality multi-winding components that are robust enough to handle the conditions SMD components are exposed to during their production and use.
For example, in the conventional SMD transformer shown in FIG. 9, the component is constructed using a plastic bobbin (or coilform) upon which the windings of the component are wound. A problem with this configuration is that the plastic bobbins or coilforms are often times incapable of handling the extreme heat or high temperatures the component is exposed to during its manufacturing. A particular drawback to this type of component is the amount of warping or deformation the bobbin experiences when the component covered PCB (or populated PCB) passes through the re-flow oven in order to create the electrical connection between the component and the PCB. During this solder reflow stage, the populated PCB is heated to a high enough temperature (e.g., 200xc2x0 C.-260xc2x0 C.) to heat the metalized pads of the component and the corresponding lands on the PCB and to liquify the solder paste therebetween so that an electrical connection (or solder joint) can be established between the metalized pads and lands once the solder lowers in temperature. Often times, this temperature increase is enough to deform or warp the plastic bobbin causing the component and its solder joint to incur unwanted stress due to the deformation. Such deformations or warping may prevent the component from retaining its low profile shape or desired height from the surface of the PCB, and may cause the component or circuit to experience failures over their lifetime. For example, warping may induce enough strain on the solder joint to actually lift the land or solder pad and trace up from the PCB. Such an act can cause the trace or solder joint to break away formt he PCB creating an open circuit condition or a condition in which the circuit may only work intermittently.
In order to reduce the risk of such warping or deformation, the solder reflow stage could be conducted at a lower temperature; however, such an adjustment may result in the metalized pads, lands, and/or solder paste failing to reach a sufficient temperature to make a solid electrical and mechanical connection to the PCB. For example, if the metalized pad of the component does not heat to a sufficient temperature it may not bond with the melted solder paste causing a cold solder joint to be formed and resulting in either a poor/intermittent electrical connection between that pad of the component and its corresponding land on the PCB, or an open circuit condition in the circuit of the PCB.
Another drawback to using plastic bobbins for multi-winding components is that the component typically is required to use terminal pins extending out from the body of the component, thereby increasing the overall amount of space needed for the component. Given that the current desire in the industry is to make smaller components and smaller circuits, this increase in the space requirement for the component may make the component impractical for certain applications. Moreover, by having terminal pins extending from its side, the component leaves exposed current carrying coils and pins which can be shorted together by loose fragments within the circuit housing and/or inadvertently touched by individuals servicing or testing the electronic circuit. Thus, such a configuration allows for the component and circuit to be damaged, and increases the risk of electrical shock.
Although the terminal pins of the component of FIG. 9 are shielded by its core halves when assembled, other components using terminal pin configurations do not shield the exposed coil windings of the terminal pins which can increase the amount of noise, such as electromagnetic interference (EMI) and/or radio frequency interference (RFI), caused by the component. For example, with current running through the exposed coil windings, the electric or magnetic lines of flux of the component will be widely disbursed about the component. This increases the likelihood of the component causing interference with other components in the circuit and prevents the component from operating as optimally as it can due to disbursed flux lines.
The use of terminal pins also increases the cost for manufacturing the component because it requires the wire from the windings to be wound about the terminal pins and then dipped into a solder pool or bath, (i.e., dip soldering), in order to remove the wire insulation and create an electrical connection or solder joint between the wire winding and the terminal pin of the component. The need for additional equipment and/or manual labor to hand wind the component increases the cost of the component and makes it less likely to be used in a number of applications. Furthermore, when the component is dip soldered, the plastic bobbin is again exposed to high temperatures which may result in further warping or deformations.
Another problem associated with the shaped core and bobbin configuration of FIG. 9, is that it does not have a seamless flat top portion for allowing industry standard pick-and-place equipment to position the component on the PCB, and thus does not have a configuration that is easy to implement into the traditional tape and reel carrier format used by a majority of the electronics industry. Such a configuration also increases the cost of manufacturing the overall circuit by requiring specialized equipment for placement of the component, or by requiring manual placement of the component, which increases the amount of time and cost needed to fully assembly the circuit, making the component less likely to be used in a majority of applications.
A solution to several of the problems associated with plastic bobbins was created by Coilcraft, Incorporated of Cary, Ill., which involved replacing the plastic bobbin/terminal pin configuration with a component having a ceramic base, a core made of a magnetic material, and a flat top portion made of acrylic. As shown in FIGS. 10A-B, metalized pads are capable of being bonded directly to the ceramic base of the component so that the ends of the wire winding can be electrically connected to the component and the component can be electrically and mechanically connected to the PCB. This allows the component""s terminals or metalized pads to be positioned below the component, without exposure of the wire winding, and also prevents the terminals or pads from increasing the overall size of the component (or amount of space the component takes up).
Further, the use of a base material having a high temperature tolerance, such as ceramic, allows the component to withstand the high temperatures of the solder reflow stage mentioned above (e.g., 200xc2x0 C.-260xc2x0 C.) without experiencing the warping or deformation that a plastic bobbin is subject to. Such a configuration also allows for mechanically strong materials such as ceramic to be used for the component making the device better equipped to handle the stresses and shocks it is likely to experience over its lifetime of use.
The configuration of the component of FIGS. 10A-B does not provide optimal shielding of the component or optimize the component operation by concentrating the flux lines of the component. The acrylic top portion provides a seamless flat surface with which the component can be positioned using traditional component placement equipment, but it does not provide the desired shielding of the component and/or fails to improve the electrical performance of the component by further concentrating the flux lines of the component. These problems have attempted to be solved via use of a cover made out of magnetic material such as that shown in FIGS. 11A-B, however, difficulties have been incurred in trying to keep a desired amount of distance between the cover and the winding or core. For example, with respect to the cover of the component from FIGS. 11A-B, the cover may inadvertently be placed into contact with the winding of the component.
Furthermore, the component of FIGS. 10A-B and 11A-B are not configured for handling multi-winding components consisting of windings of three or more wires. With the components only having four metalized pads, and each wire ending needing its own metalized pad, the components of FIGS. 10A-B and 11A-B cannot be configured with windings of more than two wires. Moreover, these components do not provide their manufacturer with the ability to use the same base structure for several different multi-winding components (e.g., windings of two separate wires, three separate wires, four separate wires, etc.). For example, the base of FIGS. 10A-B cannot be used to manufacture a multi-winding component having a winding consisting of two separate wires in one instance, and a multi-winding component having a winding of three separate wires in another instance. Nor do these components offer their manufacturer the ability to wire the components so that a wire""s ends can be bonded to a variety of different terminal pads if desired. For example, if a winding of two separate wires was desired, in the component of FIGS. 10A-B the wires would be wound with the ends of the wire being bonded to metalized pads located near one another. In alternate embodiments, however, it may be desirable to distance the space between the wire ends, e.g., to meet a customer""s desired land foot print on the PCB.
Accordingly, it has been determined that the need exists for a surface mountable electronic component having an improved structure for increasing the performance of the component and which overcomes the aforementioned limitations and further provides capabilities, features and functions, not available in current devices.
A low profile electronic component in accordance with the invention includes an elongated core, which is connected to a base having a plurality of metalized pads attached thereto for electrically and mechanically connecting the component to a printed circuit board. The component also includes support structures or spacers which are positioned at the ends of the core and, in combination with the core, serve to shield the component from interference and concentrate the magnetic lines of flux emitted by the component in order to increase the flux density and inductance of the component.
The component also includes a winding of wire wound about at least a portion of the base and core assembly between the supports, and has the ends of the wire electrically and mechanically connected to the metalized pads of the base. By positioning the winding of wire between the supports, the magnetic lines of flux of the component are condensed into a tighter concentration causing the flux density and inductance of the component to increase.
A top portion or shielding structure may be connected to the core via the supports (or spacers) in order to cover at least a portion of the windings of wire of the component to further shield the component. The supports separate the core and the top portion and maintain the top portion at a desired position with respect to the winding and the core. The top portion also concentrates the magnetic lines of flux of the component, thereby increasing its flux density and inductance. As such, the core, supports, and top portion provide a source of additional shielding for the component thereby reducing the amount of electro-magnetic interference and/or radio frequency interference caused by the component when mounted in a circuit on the PCB. This improves the performance of the component and optimizes it for use in a variety of applications.
In one embodiment the supports form an integral part of the core and are therefore made of the same material as the core. The base and core form an I-shaped assembly to which the top portion may be attached. In another embodiment, the supports form an integral part of the top portion and are made of the same material as the top portion. In yet another embodiment, the supports may comprise their own structure rather than being integral to the core or top portion and may be made of similar material to the core or top portion, or each component (the core, top portion and supports) may be made of different materials altogether.
A component made in accordance with the invention may also contain insulators for isolating the core from the top portion. In alternate embodiments, the insulators may comprise a portion of the supports or make up the entire support in and of itself. The insulators provide a gap between the top portion and the core which may be desirable in certain multi-winding components.